CN115916736A

CN115916736A - Preparation of saturated aliphatic C 6-12 Process for preparing carboxylic acids

Info

Publication number: CN115916736A
Application number: CN202180050822.9A
Authority: CN
Inventors: J·N·哈曼; J·H·特莱斯; P·G·奥勒穆勒; F·盖奇; 梁社略; S·里廷格; F·特龙; R·德恩
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2020-08-20
Filing date: 2021-08-09
Publication date: 2023-04-04
Also published as: JP2023541799A; BR112023002841A2; US20240010597A1; WO2022037978A1; EP4200273A1; EP4200273B1

Abstract

Preparation of color-stable saturated aliphatic C _6‑12 A process for preparing a carboxylic acid, wherein (1) a crude saturated aliphatic carboxylic acid in a liquid mixture is obtained by oxidation of the corresponding aldehyde with molecular oxygen, (2) molecular oxygen is removed from the crude saturated aliphatic carboxylic acid mixture, and (3) the saturated aliphatic carboxylic acid is separated as a color-stable product from the molecular oxygen-depleted mixture by distillation.

Description

Preparation of saturated aliphatic C 6-12 Process for preparing carboxylic acids

The invention relates to a method for producing saturated aliphatic carboxylic acids having 6 to 12C atoms by oxidizing the corresponding aldehydes with molecular oxygen, wherein the saturated aliphatic carboxylic acids are obtained in high purity by tempering under inert gas conditions at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours with a very low tendency to darken.

Furthermore, the invention relates to 2-ethylhexanoic acid having a very low tendency to darken by tempering under inert gas conditions for 4 hours at a temperature of 225 ℃ and a pressure of 0.1 MPa.

Saturated aliphatic carboxylic acids are important intermediates in a wide range of applications worldwide. They can be used as such, but are usually further processed to metal salts, esters, amides, anhydrides, acid chlorides and other derivatives. In general, they are important intermediates for the production of various compounds, such as metal salts and metal soaps, flavorings, fragrances, pharmaceutical and agrochemical ingredients, cosmetic ingredients, plasticizers, paints, coating additives, coolants, lubricants or catalysts for polymer processing. Saturated aliphatic C _6-12 A very important representative of carboxylic acids is 2-ethylhexanoic acid. It is used mainly in the form of its derivatives, such as its metal salts or esters, as drying agents and thickeners for alkyd resins and paints, as catalysts in the manufacture of polyurethane foams, as PVC stabilizers and/or plasticizers, or as anti-wear agents and corrosion inhibitors for lubricants. Saturated aliphatic C _6-12 Esters of carboxylic acids such as n-heptanoic acid, n-octanoic acid, 2-ethylhexanoic acid, n-nonanoic acid or 3,5,5-trimethylhexanoic acid are also frequently used as lubricants.

An important widely used method for producing saturated aliphatic carboxylic acids having 6 to 12 carbon atoms is in the presence or absence of a saturated aliphatic carboxylic acid in a liquid phaseThe corresponding aldehyde is oxidized with molecular oxygen under a catalyst or any additive. This general synthetic route is described, for example, in J.Kubitschke et al, "Carboxylic acids, aliphatic", ullmann's Encyclopedia of Industrial Chemistry,2014, wiley-VCH Verlag GmbH&KGaA, DOI 10.1002/14356007.05 u 235.Pub2, chapter 4.2.1 "Aldehydeoxidation". Saturated aliphatic C obtained by the above-mentioned oxidation _6-12 The carboxylic acid is then usually purified by distillation to obtain the saturated aliphatic C in preferably pure form _6-12 A carboxylic acid.

Since most applications require highly transparent and colorless products, not only high chemical purity, preferably significantly above 99% by weight, but also very low APHA color numbers are required.

US 5,504,229 describes the preparation of 2-ethylhexanoic acid by oxidation of 2-ethylhexanoic aldehyde in the presence of potassium 2-ethylhexanoate as selectivity improving additive and subsequent distillation to obtain purified 2-ethylhexanoic acid, wherein the potassium 2-ethylhexanoate is enriched in the bottom product and recycled to the oxidation stage. The 2-ethylhexanoic acid obtained shows a low APHA color number of 4 according to example 1.

CN 109438216 discloses the preparation of 2-ethylhexanoic acid by a multi-step process in which n-butyraldehyde is subjected to aldol condensation to obtain 2-ethyl-3-hexanal, which is subsequently hydrogenated to 2-ethylhexanal. The inventors realized that after the hydrogenation step a small amount of 2-ethyl-3-hexanal remains, which would be oxidized in the oxidation step together with the 2-ethylhexanal, resulting in crude 2-ethylhexanoic acid contaminated with 2-ethyl-3-hexenoic acid, which again could not be separated from the 2-ethylhexanoic acid by distillation. CN 109438216 teaches the conversion of 2-ethyl-3-hexanal remaining after hydrogenation in the presence of an acidic catalyst to 2-ethyl-3-hydroxyhexanal, which can be oxidized to 2-ethyl-3-hydroxyhexanoic acid in an oxidation step, which can be separated from 2-ethylhexanoic acid by distillation. According to examples 1 to 5, 2-ethylhexanoic acid having a purity of up to 99.91% by weight and a low APHA color number of up to 3 can be obtained.

According to the invention, a new distilled saturated aliphatic C is recognized _6-12 The low APHA color number of carboxylic acids does not guarantee that this low APHA color number remains unchanged over a longer period of storage, nor does it guaranteeThe corresponding carboxylic acids are used in color stable products. Saturated aliphatic C is considered _6-12 Carboxylic acids generally tend to darken over time when subjected to thermal stress during storage and/or in products in which they are conventionally used. Furthermore, it is recognized according to the present invention that such darkening may be caused by the presence of peroxide.

It is known from the prior art to oxidize aldehydes with oxygen initially to form peroxy acids, which are subsequently oxidized further to yield 2 moles of carboxylic acid per mole of intermediate peroxy acid. This mechanism is described, for example, in J.H.Teles et al, "Oxidation", ullmann's Encyclopedia of Industrial Chemistry,2015, wiley-VCH Verlag GmbH&KGaA, DOI 10.1002/14356007. 18 u 261.Pub2, section 5.4.1 "Secondary reactions of radials, peroxides, and other intermediates" and section 2.2.8 "Carboxyic acids, processed". It is therefore clear that the peroxy acids also appear as intermediates in the oxidation of the corresponding aldehydes by oxygen to the corresponding saturated aliphatic C _6-12 In the process of carboxylic acids. Peroxy acids are extremely reactive molecules. Saturated aliphatic C's even if only small amounts of them remain in the crude carboxylic acid product after oxidation and are not properly separated _6-12 Carboxylic acids may also lead to undesirable properties, such as poor color stability.

CN 108047027 relates to the decomposition of peroxide formed in the oxidation of 3,5,5-trimethylhexanal to 3,5,5-trimethylhexanoic acid (isononanoic acid). It was found that although its concentration is rather low, it is still high enough to cause problems in the rectification column, since peroxide may accumulate therein due to the difference in boiling point of the peroxy acid and other components. Furthermore, the chinese application describes the use of lower concentrations of homogeneous catalysts to decompose peroxides in the prior art. However, such homogeneous catalysts are very difficult to separate out and also cause the risk of slagging, plugging and explosion in the distillation column. To avoid such problems, the chinese application teaches heterogeneously catalyzing the decomposition of peroxide over a metal organic framework catalyst prior to entering the rectification column. It is taught to be of critical importance that the peroxide is decomposed only at low temperatures of 20-70 c, in order that side reactions such as decarboxylation would otherwise occur and reduce product and purity. In addition, to be emphasizedIt is that the decomposition is very fast and can therefore be between 5 and 40h ^-1 Which involves short residence times of 1.5 to 12 minutes.

However, it is often disadvantageous to use metal organic framework catalysts as peroxide decomposition catalysts. First, these metal organic framework catalysts are very complex to produce. Secondly, the organic molecules constituting the backbone are susceptible to oxidation, especially in the presence of peroxy acids and the reactive groups generated in their decomposition. In the presence of carboxylic acids, the metal organic framework is known to lose activity and/or the framework metal due to leaching. These leached metals subsequently cause the same problems in the rectification column as the metals used as homogeneous catalysts. Furthermore, all these factors lead to a short lifetime of the metal organic framework catalyst and thus to an additional increase in the complexity of the process by disposing of the spent catalyst and providing a new catalyst.

PCT application No. PCT/EP2020/087,952 (based on the priority of EP application No. 20150845.4) recognizes saturated aliphatic C _3-5 C in crude Carboxylic acid product _3-5 The peroxy acids remaining after oxidation of the aldehydes can be decomposed mainly by thermal treatment before distillative purification. Distilled saturated aliphatic C _3-5 Carboxylic acids show only very low active oxygen content, which in turn indicates very low levels of peroxides such as peroxy acids. The active oxygen content is a quantitative measure of the amount of reactive oxygen capable of oxidizing an oxidizable compound, such as the salt of iodide (1-), to iodine (0) or the salt of iron (II) to iron (III).

It is recognized according to the invention that the process proposed in European patent application No. 20150845.4 for the oxidation of the corresponding C _3-5 Production of saturated aliphatic C from aldehydes _3-5 Such heat treatment of carboxylic acids does not involve oxidation of the corresponding C _6-12 Successful aldehyde production of saturated aliphatic C _6-12 Carboxylic acids, at least not within a reasonable time of only a few hours. Albeit possibly C _6-12 Peroxyacids may decompose by such heat treatment and distilled saturated aliphatic C _6-12 Carboxylic acids exhibit low APHA color numbers, but their active oxygen content is still high, and they tend to darken over time when subjected to thermal stress during storage and in products in which they are conventionally used.

It was therefore an object of the present invention to find a process for preparing saturated aliphatic carboxylic acids having from 6 to 12 carbon atoms by oxidation of the corresponding aldehydes with oxygen, which enables the corresponding saturated aliphatic carboxylic acids to be produced in high yields and high purity, in particular during storage, when the product is subjected to thermal stress and/or after its customary use (for example as an additive in the preparation of polymers), for example without tendency or at least with very low tendency to darken over time. The process should also be easy to handle, safe in performance and stable in operation over a long period of time, so as to produce the saturated aliphatic carboxylic acid at a constant high quality.

We have surprisingly found a process for preparing saturated aliphatic carboxylic acids having from 6 to 12 carbon atoms by oxidation of the corresponding aldehyde with molecular oxygen, which comprises:

(a) Converting the corresponding aldehyde with molecular oxygen at a temperature of from 0 to 120 ℃ and an oxygen partial pressure of from 0.02 to 2MPa to obtain a liquid mixture containing the saturated aliphatic carboxylic acid and less than or equal to 2mol% of the corresponding aldehyde relative to the saturated aliphatic carboxylic acid and molecular oxygen;

(b) Removing molecular oxygen from the liquid mixture obtained in step (a) to an amount of 10 ppm by weight or less based on the liquid mixture; and

(c) Distilling the mixture obtained in step (b) in a distillation apparatus comprising a purification column and withdrawing therefrom a purified distillate containing 95% by weight or more, based on the distillate, of the saturated aliphatic carboxylic acid.

Hereinafter abbreviated to saturated aliphatic C _6-12 The saturated aliphatic carboxylic acids having 6 to 12 carbon atoms to which the carboxylic acids and the production processes found are very suitable can be linear or branched and substituted or unsubstituted. Substituted saturated aliphatic C _6-12 Carboxylic acids contain one or more heteroatoms in addition to carbon and hydrogen, and halogen is mentioned as an example thereof. Unsubstituted saturated aliphatic C _6-12 Carboxylic acids are preferred. Preferred examples divided by the number of carbon atoms thereof are

For C ₆ Hexanoic acid, 2-methylvaleric acid, 3-methylvaleric acid, 4-methylvaleric acid, 2,3-dimethylbutyric acid, and 3,3-dimethylbutyric acid;

for C ₇ Heptanoic acidAnd 2-methylhexanoic acid;

for C ₈ Octanoic acid, 2-methylheptanoic acid, 2-ethylhexanoic acid, 2-ethyl-4-methylpentanoic acid and 2-propylpentanoic acid;

for C ₉ Nonanoic acid and 3,5,5-trimethylhexanoic acid;

for C ₁₀ Decanoic acid, 2-propylheptanoic acid and 2-propyl-4-methylhexanoic acid;

for C ₁₁ Undecanoic acid and 2-methyldecanoic acid; and

for C ₁₂ Dodecanoic acid and 2-butyl octanoic acid.

Of the above list, hexanoic acid, 2-methylpentanoic acid, heptanoic acid, octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid, and dodecanoic acid are more preferred. Particularly preferred is saturated aliphatic C _8-12 Carboxylic acids and among them octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid and dodecanoic acid. Very particular preference is given to 2-ethylhexanoic acid.

In the first step of the process of the invention, designated as step (a), the corresponding aldehyde is oxidized by oxygen. With respect to the more preferable saturated aliphatic C _6-12 Carboxylic acids, these aldehydes are n-hexanal for hexanoic acid, 2-methylpentanal for 2-methylpentanoic acid, n-heptanal for heptanoic acid, n-octanal for octanoic acid, 2-ethylhexanal for 2-ethylhexanoic acid, n-nonanal for nonanoic acid, 3,5,5-trimethylhexanal for 3,5,5-trimethylhexanoic acid, n-decanal for decanoic acid, 2-propylheptanal for 2-propylheptanoic acid, and n-dodecanal for dodecanoic acid.

C _6-12 Aldehydes can generally be readily prepared by a variety of methods, depending on the accessibility of the primary product. A typical preparation is the hydroformylation of the corresponding olefin having one carbon atom less than the desired aldehyde. For example, the hydroformylation of 1-pentene to n-hexanal is mentioned. Another typical preparation is the condensation of aldols to enals as intermediates, which are then hydrogenated to the desired aldehydes. For example, mention is made of the aldol condensation of n-butyraldehyde to 2-ethyl-2-hexanal and its subsequent hydrogenation to 2-ethylhexanal. Last but not least, the third exemplary preparation process is the correspondingDehydrogenation of alcohol. For this process, dehydrogenation of 1-decanol to n-decanal is exemplified.

The aldehyde to be oxidized can be used in diluted or pure form. If the aldehyde is used in diluted form, the diluent should preferably be a compound which is inert to oxidation with oxygen, stable to the carboxylic acid produced and easily separable from the carboxylic acid by distillation. The above-mentioned necessity of separating the diluent from the carboxylic acid can be avoided if the corresponding carboxylic acid is used as diluent. However, it is preferred not to dilute the aldehyde deliberately, since the diluent increases the reaction volume and thus reduces the space-time yield and additionally may cause contamination of the desired carboxylic acid if other diluents than the corresponding carboxylic acid are used. The aldehydes are preferably used as highly concentrated compounds having an aldehyde content of preferably 80 to 100% by weight, more preferably 80 to 100% by weight, particularly preferably 95 to 100% by weight, very particularly preferably 99 to 100% by weight.

In addition to the preparation of only one specific saturated aliphatic carboxylic acid, the process of the present invention can also be used to prepare saturated aliphatic C _6-12 A mixture of carboxylic acids. If such a mixture is prepared, saturated aliphatic C's having the same number of carbon atoms are particularly preferred _6-12 A mixture of carboxylic acids. However, it is generally more preferred to prepare unmixed saturated aliphatic C' s _6-12 A carboxylic acid.

Particular preference is given to preparing 2-ethylhexanoic acid by oxidizing 2-ethylhexanoal.

The oxidation of the aldehyde is carried out with molecular oxygen. It can be used in pure form or diluted with other gases, e.g. in air, O ₂ /N ₂ In the form of a mixture or in the form of a mixture with other inert gases.

The oxidation reaction may be carried out in the presence or absence of an oxidation catalyst and/or in the presence or absence of a selectivity-improving additive. If oxidation catalysts are used, they are homogeneous catalysts. As examples of homogeneous oxidation catalysts are mentioned salts of transition metals of groups 6 to 11 of the periodic Table of the elements, preferably salts of the first row of these groups, most preferably Mn, fe or Co salts. If selectivity improving additives are used, they are also homogeneous. Mention may be made, as examples of selectivity-improving additives, of salts of alkali metals, alkaline earth metals and transition metals of group 12 of the periodic Table of the elements, preferably of Na, K, mg, ca, zn or Cd, most preferably of K or Na, and particularly preferably of K. These salts may be selected from any salt that is soluble in the reaction mixture, but carboxylates, hydroxides, carbonates and bicarbonates are preferred. The concentration of the homogeneous oxidation catalyst metal can vary within wide ranges, but metal contents of 0.0001 to 0.1% by weight, based on the reaction mixture, are typical. The concentration of the homogeneous selectivity-improving metal can also vary within wide limits, but a metal content of from 0.01 to 5% by weight, preferably > 0.02% by weight, more preferably > 0.05% by weight, based on the reaction mixture, preferably < 2% by weight, more preferably < 1% by weight, particularly preferably < 0.5% by weight, based on the reaction mixture, is a typical content. It is also possible to use both homogeneous catalyst metals and homogeneous selectivity improving metals.

Depending on the nature of the aldehyde, the presence of the oxidation catalyst and in particular the presence of the selectivity improving metal will affect the range and type of by-products. For example, if no selectivity improving metal is present, α -branched aldehydes such as 2-ethylhexanal tend to produce more formate esters as a by-product. However, in the presence of selectivity improving metals, especially in the presence of sodium or potassium salts, most especially in the presence of potassium salts, the α -branched aldehydes produce much less of the undesired formate esters. Thus, it is preferred that the alpha-branched aldehyde is oxidized in the presence of a selectivity improving metal, preferably in the presence of a sodium or potassium salt, particularly preferably in the presence of a potassium salt. On the other hand, linear aldehydes such as n-hexanal or n-decanal already form only very small amounts of formates in the absence of selectivity improving metals, and thus the addition of selectivity improving metals is not relevant or only less relevant for the selectivity. Therefore, it is preferred that the linear aldehyde is oxidized in the absence of a selectivity-improving metal.

For homogeneous catalytic metals, they increase the reaction rate, but they have a detrimental effect on selectivity. Thus, it is preferred to carry out the oxidation in the absence of added homogeneous catalyst metal.

The oxidation reaction is carried out at a temperature of 0 to 120 ℃ and an oxygen partial pressure of 0.02 to 2MPa, irrespective of the presence or absence of an oxidation catalyst. Preference is given toAt a temperature of 10 ℃ or more, more preferably 20 ℃ or more, particularly preferably 30 ℃ or more and preferably 100 ℃ or less, more preferably 80 ℃ or less, particularly preferably 60 ℃ or less. As for the oxygen partial pressure, it is preferably carried out at an oxygen partial pressure of not less than 0.05MPa, more preferably not less than 0.1MPa, particularly preferably not less than 0.11MPa and preferably not more than 1.5MPa, more preferably not more than 1 MPa. The oxygen partial pressure may be determined by measuring the total pressure and multiplying by the O determined in volume percent by any suitable method known in the art ₂ The concentration can be easily measured.

Although the oxygen partial pressure may vary within wide ranges based on the oxygen content of the oxygen source, the total pressure in step (a) is typically in the range of 0.01 to 5MPa abs. The oxidation reaction is preferably carried out at a total pressure of 0.1MPa abs or more, more preferably 0.2MPa abs or more. Preferably at a total pressure of 4MPa abs or less, more preferably 3MPa abs or less.

Under these conditions, the aldehyde is almost completely in the liquid phase and the oxidation reaction also proceeds in the liquid phase.

The oxidation of the aldehyde in step (a) is generally carried out batchwise, semicontinuously or continuously in the reaction apparatus. In continuous operation, aldehyde and oxygen are continuously fed into the reaction apparatus and a sufficient stream of the reaction mixture is continuously withdrawn. The process conditions, including residence time, for continuous operation are selected in such a way that the desired conversion is achieved. In batch operation, the reaction apparatus is charged and, if necessary, supplemented with added aldehyde and oxygen. After the desired conversion has been achieved, the mixture is removed from the reaction apparatus. The semi-continuous operation is characterized in that aldehyde and oxygen are fed into the reaction apparatus together or intermittently over a specified period of time while the oxidation reaction has taken place. After a certain time, for example if the reaction apparatus is more or less filled, the feed is stopped and the mixture is removed from the reaction apparatus after the desired conversion has been achieved.

Preferred operations of step (a) are batch and continuous operations, with continuous operations being particularly preferred.

The reaction apparatus in which the oxidation of the aldehyde in step (a) is carried out may comprise one or more reaction devices. Suitable reaction apparatus in principle include apparatus which is suitable for carrying out gas-liquid exothermic reactions and which can be operated discontinuously, semicontinuously or continuously. For discontinuous processes, stirred autoclaves or autoclaves with jet circulation mixing are suitable, for example. For the semi-continuous process, stirred vessels, trickle bed reactors and bubble column reactors are mentioned as possible examples. For continuous processes, stirred vessels, trickle bed reactors, bubble column reactors, jet loop reactors and cascades of the above mentioned reactors are mentioned as suitable examples. Preferred examples of suitable reactors are described in more detail in WO 2009/024,446 and WO 2009/024,549. If a reactor cascade is used, as is the case, for example, in a continuous process, 2 to 5, preferably 2 to 4, particularly preferably 2 to 3 reactors are connected in series.

Preference is given to using reaction equipment which allows for a sufficient gas-liquid mixing and a good distribution of oxygen in the liquid reaction mixture.

Since a significant amount of reaction heat is formed by this oxidation, heat needs to be removed from the reaction zone. Depending on the aldehyde and oxygen concentrations fed to the reactor, it is sufficient in a continuously carried out process to remove heat only with the reaction mixture and to control the temperature in the reactor by adding fresh aldehyde having a low temperature. However, for higher concentrations of aldehydes and at air levels or higher oxygen levels, it is generally required to cool the reaction liquid in the reactor. The cooling can be effected, for example, by means of an externally cooled outer wall of the reactor or by means of cooling tubes in the reactor through which a coolant flows or by using an external heat exchanger in an external circuit.

According to the general chemical equation:

wherein R represents C _5-11 Aldehydes and C _5-11 C of carboxylic acids _5-11 The oxidation of the aldehyde to the carboxylic acid requires 0.5mol oxygen O ₂ . Although it is possible to carry out the oxidation under oxygen-with partial conversion and the presence of residual aldehyde in the reaction mixture, it is preferred to use oxygen in stoichiometric or superstoichiometric amounts. To is coming toOn the one hand to ensure sufficient conversion and on the other hand to limit the gas load, the oxidation reaction is preferably carried out at an oxygen/aldehyde molar ratio of from 0.5 to 1. More preferably at an oxygen/aldehyde molar ratio of ≥ 0.51, particularly preferably ≥ 0.52 and more preferably ≤ 0.7, particularly preferably ≤ 0.6 and very particularly preferably ≤ 0.58.

The use of a reactor cascade has been shown to be particularly advantageous for a continuous oxidation process, since it allows a stepwise addition of oxygen. The great advantage of this stepwise addition of oxygen is better thermal control of the reaction, in particular a smaller gas proportion in the respective cascade stage. It is therefore particularly preferred to use a reactor cascade of from 2 to 3 reactors, whereby preferably from about 70 to 95% of the total amount of the aldehyde and of the total oxygen are fed to the first reactor and the remaining 5 to 30% of the oxygen are either fed in total to the second reactor or are fed further in two parts to the second and third reactor, wherein the proportion of the third reactor will then preferably be a smaller proportion.

Even if oxygen is applied in stoichiometric or superstoichiometric amounts, it takes a very long time to achieve almost 100% conversion of aldehyde. This would unnecessarily plug the reaction equipment or make it very large. It is therefore advantageous to convert the aldehyde in step (a) until a residual aldehyde content of < 2mol% relative to the saturated aliphatic carboxylic acid is achieved. Depending on the nature of the aldehyde, the concentration of the oxygen-containing gas supplied to the reaction apparatus and the process conditions, a residual aldehyde content of < 2mol% relative to the saturated aliphatic carboxylic acid is generally achieved after a reaction time of from 0.1 to 5 hours.

The composition of the reaction mixture of step (a) can generally be determined by gas chromatography with respect to the content of the saturated aliphatic carboxylic acid and the corresponding aldehyde.

The mixture obtained in step (a) preferably contains 1.5mol% or less, more preferably 1mol% or less, particularly preferably 0.5mol% or less, very particularly preferably 0.3mol% or less and preferably 0.05mol% or more, particularly preferably 0.1mol% or more of the corresponding aldehyde relative to the saturated aliphatic carboxylic acid.

In terms of reaction time, a time of 0.2 hours or more is preferred, a time of 0.3 hours or more is more preferred, and a time of 0.5 hours or more is particularly preferred. Furthermore, a time of 8 hours or less is preferred, 4 hours or less is more preferred, and 3 hours or less is particularly preferred.

After a mixture containing a saturated aliphatic carboxylic acid and 2mol% or less, relative to the above-mentioned saturated aliphatic carboxylic acid, of the corresponding aldehyde is obtained in step (a), the reaction liquid is preferably then separated from the remaining oxygen-containing gas phase. In a batch process this can be achieved, for example, by simply withdrawing an oxygen-containing gas phase, whereas in a continuous process this can be achieved, for example, by simply withdrawing a liquid reaction mixture from the reaction apparatus.

With regard to possible by-products of the separated reaction liquid, it is known from the prior art that peroxy acids are initially formed in the oxidation of aldehydes with oxygen, followed by further oxidation of the aldehydes to yield 2 moles of carboxylic acid per mole of intermediate peroxy acid. The reaction procedure is as follows

Wherein R represents C _5-11 A group. It is expected that such acids will also be useful in the preparation of saturated aliphatic C' s _6-12 The oxidation of the carboxylic acid is formed in step (a) and although the added aldehyde is generally not fully converted, it is expected to remain to some small extent in the liquid reaction mixture. Peroxy acids have a high oxidation potential and can lead to darkening of carboxylic acids over time during storage when the products are subjected to thermal stress and/or in products where they are conventionally used, if not properly isolated. However, peroxy acids, together with other components having a high oxidation potential, are readily characterized by so-called "active oxygen", which is a quantitative measure of the amount of reactive oxygen. The term "active oxygen" is known and used in the prior art and is described, for example, in A.Uhl et al, "organic peroxy compounds", ullmann's Encyclopedia of Industrial Chemistry,2017, wiley-VCH Verlag GmbH&KGaA, DOI 10.1002/14356007.A19_199.Pub2, chapter 10 "analytical assay". The amount of active oxygen in a sample is generally determined by adding a defined amount of an easily oxidizable compound, such as the iodide salt (1-) or the iron salt (II), to a defined amount of the sample. The oxidizable compound is oxidized by the active oxygen present and the oxidized oxidizable compound is then determined by titrationThe amount of (c).

Since the boiling point of peroxy acids usually does not deviate significantly from those of the corresponding acids, separation from the corresponding acids is usually not easy and the impurities based on peroxy acids as distilled acids lead to darkening over time during storage, when subjected to thermal stress and in products where acids are conventionally used, it was initially assumed that such peroxy acids also lead to saturated aliphatic C' s _6-12 The cause of darkening of carboxylic acids. Based on the preparation of saturated aliphatic C in European patent application No. 20150845.4 _3-5 Teaching of carboxylic acids, which has already been mentioned in the introductory summary of the prior art, it is expected that peroxy acids are readily decomposed by thermal treatment, which is taught to be carried out prior to distillative purification. However, it is recognized according to the invention that such heat treatment is at saturated aliphatic C _6-12 The production of carboxylic acids has not been successful, at least for a reasonable time of only a few hours. First, the saturated aliphatic C prepared according to step (a) _6-12 The active oxygen content in the crude carboxylic acid product is only slightly reduced by this heat treatment. Second, despite possible C _6-12 Saturated aliphatic C which may be decomposed and distilled by such heat treatment _6-12 Carboxylic acids exhibit a low APHA color number, but their active oxygen content is still high and tend to darken over time during storage, when subjected to thermal stress and in products in which they are conventionally used.

It has surprisingly been found that, in the subsequent distillation of the heat-treated product and also in the distillation of the non-heat-treated product, the majority of the components which cause the active oxygen content are enriched in the bottom fraction and only in the distilled saturated aliphatic C _6-12 Small but still noticeable moieties are found in carboxylic acids. These two findings lead to important conclusions, saturated aliphatic C _6-12 The active oxygen content in the carboxylic acid crude product is mainly caused by other components than the peroxyacid. Further investigations into the properties of these other components have shown that these other components are mainly alkyl hydroperoxides having the general formula (4):

wherein R is ¹ Denotes C with COOH groups _1-11 Group, R ² Is represented by C _1-10 Radicals or H, R ³ Is represented by C _1-5 A group or H, and R ¹ 、R ² And R ³ The sum of carbon atoms of (a) is from 5 to 11, depending on the aldehyde used. The hydroperoxy group may be located in a saturated aliphatic radical C _6-12 At any carbon atom of the carboxylic acid, whether the carbon atom is a primary, secondary or tertiary carbon atom, with the exception of the COOH group. However, tertiary carbon atoms, as well as carbon atoms in the alpha position of the COOH group, are particularly susceptible to peroxidation.

Alkyl hydroperoxides can cause the carboxylic acid to darken over time during storage, when subjected to thermal stress and/or in the products to which they are conventionally applied, if not properly isolated.

Alkyl hydroperoxides of saturated aliphatic carboxylic acids are known to have significantly higher boiling points than the corresponding saturated aliphatic carboxylic acids. The following table shows the boiling points of some 2-hydroperoxy acids compared with the boiling points of the corresponding acids. Their boiling points at atmospheric pressure have been estimated by the SciFinder, a Chemical and bibliographic information electronic database provided by the American Chemical Society (the American Chemical Society)

195 + -8 ℃ for 2-methylpentanoic acid;

295 +/-23 ℃ for 2-methyl-2-hydroperoxypentanoic acid;

239 ± 3 ℃ for octanoic acid;

327. + -. 25 ℃ for 2-hydroperoxycaprylic acid;

306 plus or minus 10 ℃ for 2-ethyl decanoic acid; and

373 + -25 deg.C for 2-ethyl-2-hydroperoxycanoic acid.

This significant difference in boiling points should facilitate easy separation by distillation. The high active oxygen content of the bottoms fraction found in the above investigations is also a strong indication. Based on this, however, it is even more surprising that the saturated aliphatic C in distillation _6-12 Still a small but still noticeable portion of the active oxygen can be found in the carboxylic acid. Owing to their high boiling point, slipping of the corresponding hydroperoxy acid (slippage) during the distillation is highly unlikely.

In order to more accurately understand the information value of the active oxygen content, the measurement method thereof will be described in more detail. According to the invention, the determination of the active oxygen is preferably carried out by oxidation of the iodide (1-). This method of analysis is known as iodometry and is well known to those skilled in the art. However, it is also roughly explained as follows:

in the iodometry, a defined amount of an aqueous solution of potassium iodide in acetic acid is added to a defined amount of the sample at room temperature and stirred to oxidize the iodide (1-) to elemental iodine. The amount of potassium iodide added is related to the expected amount of active oxygen and can be estimated by preliminary measures. For this iodometric measurement, the amount of iodide (1-) added must be slightly higher than the amount oxidized to elemental iodine. Elemental iodine was then titrated with sodium thiosulfate to determine the amount of elemental iodine formed from the previous oxidation. As indicator, starch is generally used, which is purple in the presence of elemental iodine and becomes colorless when all elemental iodine is reduced to iodide. Alternatively, a platinum electrode may also be used. The amount of oxidized iodide (1-) can be calculated based on the amount of potassium iodide added and the amount of elemental iodine formed by oxidation. According to the formal equation:

2I ^- +2H ⁺ +"O"→I ₂ +H ₂ O (5)

and more detailed equations for peroxy acids and alkyl hydroperoxides:

2 moles of iodide (1-) are reacted with 1 mole of active oxygen atoms in the presence of acetic acid. The active oxygen is represented by "O" in formula (5) and is a part of the peroxy group in formulae (6) and (7). It is reduced to water. In equations (6) and (7), "Ac" represents an acetyl group and the group R, R ¹ 、R ² And R ³ Have the meanings as described for formulae (2)/(3) and (4), respectively. The active oxygen content of the sample is the weight fraction of active oxygen atoms based on the weight of the sample and is expressed in weight% or weight ppm.

Active oxygen content canAt an equivalent content which is easily converted into a peroxide compound, it is assumed that active oxygen is incorporated only as a hydroperoxide in a saturated aliphatic C _6-12 In a carboxylic acid. The conversion can be made by multiplying the measured active oxygen content by the ratio of the molar mass of the hydroperoxy acid to the molar mass of oxygen atoms. For example, for 2-ethylhexanoic acid, the multiplication factor is 176.2/16.0=11.0125.

For the sake of completeness, it should also be mentioned that the amount of active oxygen in the carboxylic acid-containing sample can also be determined essentially by physical methods such as ¹³ C-NMR measurement.

In the present invention, however, active oxygen is understood to be the mass of oxygen present in the sample that is capable of oxidizing iodide (1-) to elemental iodine in an aqueous acetic acid medium at room temperature and atmospheric pressure.

The active oxygen content of the mixture obtained in step (a) is generally from 0.02 to 1% by weight, preferably ≥ 0.03% by weight, more preferably ≥ 0.05% by weight and preferably ≤ 0.8% by weight, more preferably ≤ 0.5% by weight, based on the mixture.

In the para-saturated aliphatic C _6-12 After intensive investigations into the surprising behaviour of crude carboxylic acid product mixtures, it was surprisingly found that the presence of molecular oxygen in the crude product mixture leads primarily to distilled saturated aliphatic C _6-12 Significant active oxygen content in carboxylic acids.

In the oxidation step (a), a portion of the molecular oxygen, depending on its partial pressure, the temperature of the liquid mixture and the chemical composition of the liquid mixture, remains unreacted in the liquid mixture as physically dissolved molecular oxygen. The concentration thereof in the liquid mixture obtained in step (a) is generally >10 ppm by weight to ≦ 1ppm by weight, based on the liquid mixture. A content of < 10 ppm by weight based on the liquid mixture is generally not achieved, since small amounts of molecular oxygen remain unreacted in the liquid mixture, and a content of > 1% by weight cannot be achieved in practice because of its limited solubility. Since the above-mentioned concentration of molecular oxygen is related to the liquid mixture obtained in step (a), this value refers to the reaction conditions at the end of the conversion before the liquid mixture leaves the reaction apparatus. Therefore, the measurement of the concentration of molecular oxygen in the liquid mixture is preferably carried out under the above-mentioned conditions immediately before the liquid mixture leaves the reaction apparatus. However, it may alternatively be measured at a lower pressure, for example at atmospheric pressure, the measured value being corrected taking into account a recognized law (for example the pressure-dependent henry law). The concentration of molecular oxygen in the liquid mixture obtained in step (a) is preferably ≥ 20 wt ppm, more preferably ≥ 50 wt ppm. As regards its upper limit, it is generally less than or equal to 1% by weight (or less than or equal to 10000 ppm by weight, expressed on the ppm scale), preferably less than or equal to 5000 ppm by weight, more preferably less than or equal to 2500 ppm by weight, particularly preferably less than or equal to 1000 ppm by weight, very particularly preferably less than or equal to 750 ppm by weight.

The concentration of molecular oxygen in the liquid mixture obtained in step (a) can be easily determined. Useful devices are, for example, optical sensors which are frequently used for water analysis. As one possible example, optical fluorescence sensors are mentioned. Optical fluorescence sensors are prior art and the person skilled in the art knows how to calibrate and use them. Such sensors are very specific to oxygen and very sensitive to oxygen. They can measure even very low concentrations down to 0.01 weight ppm. Since such optical fluorescence sensors are resistant to heat and pressure to some extent, they can also be used for on-line measurement.

Explains the liquid saturated aliphatic C obtained by the step (a) _6-12 The crude carboxylic acid product mixture contains hydroperoxide and at most very small amounts of the corresponding by-product, and unconverted dissolved molecular oxygen, it being noted that the term active oxygen does not include molecular oxygen. This is because molecular oxygen reacts very slowly under the conditions used to measure active oxygen in samples containing hydroperoxides, and furthermore, it is good practice to perform the measurement under an inert gas to minimize interference with molecular oxygen. As a beneficial result of this, active oxygen and molecular oxygen can be determined and evaluated separately.

Based on all the above surprising findings, it was subsequently found that during storage, when subjected to thermal stress and/or in products in which they are conventionally used, darkening of carboxylic acids can be avoided or at least significantly reduced if the liquid saturated aliphatic C obtained in step (a) is subjected to a subsequent step, referred to as step (b) _6-12 The molecular oxygen present in the crude carboxylic acid product mixture is removed to a value based onA content of less than or equal to 10 ppm by weight of the liquid mixture, in a distillation to separate saturated aliphatic C _6-12 Before the carboxylic acid. For the sake of completeness, it is mentioned that between steps (a) and (b) further steps are possible, including, for example, lowering or raising the temperature or pressure, or removing low boilers by distillation.

There are several possibilities for removing molecular oxygen from the liquid crude product mixture to a content of 10 ppm by weight or less. One possibility is to give the mixture a long time to allow the dissolved molecular oxygen to react with the aldehyde still present. This can be done, for example, in a pipe through which the mixture flows and which enables the desired residence time to be achieved, or in one or more residence time vessels connected in series. Although such a procedure does not require the addition of further compounds nor the adjustment of specific conditions (e.g. elevated temperature), it is not a preferred approach as it usually requires well over 10 hours, and even over 50 hours or even over 100 hours.

As a preferred possibility for removing molecular oxygen to a content of < 10 ppm by weight, mention is made of stripping the liquid crude product mixture obtained in step (a) with an inert gas.

Basically, the stripping can be carried out in a vessel, also referred to as a stripper, in which the liquid crude product mixture can be contacted with an inert gas to transfer dissolved molecular oxygen into the gas phase of the inert gas and remove it with the gas phase. For a continuous preparation process, the liquid crude product mixture obtained in step (a) is continuously fed to a stripper, in which inert gas can pass through the mixture. In principle, such a stripper may have different shapes, and thus in principle any kind of vessel known in the art for contacting a gas with a liquid phase may be used, but preferably a so-called stripper is preferred. The stripping column being characterized by the ratio of its length to its average diameter>1, preferably 5 or more, more preferably 8 or more, and preferably 15 or less. While inert gas is fed to the lower region of the stripper column to bubble to the upper region, the liquid crude product mixture can be fed to the lower region resulting in co-current flow or fed to the upper region resulting in counter-current flow. Preference is given to continuously preparing saturated aliphatic C under countercurrent flow by stripping in step (b) _6-12 A carboxylic acid. Whether stripping is carried out co-currently or counter-currently, it is advantageous to improve the contact between the gas and liquid phases. This can be easily achieved by random or structured packing or trays within the stripping column. Such internals for promoting gas/liquid contact are well known to the person skilled in the art and can be readily selected.

Alternatively, the molecular oxygen can also be removed by spraying the liquid crude product mixture into the top of a device having free space, such as a vertical column, in which inert gas is fed in countercurrent from the bottom.

For a discontinuous or semi-continuous production process, the liquid crude product mixture can, for example, be held in a reactor, in which the aldehyde conversion in step (a) takes place and an inert gas is passed through the liquid crude product mixture obtained, preferably after the gaseous phase of the molecular-oxygen-containing oxidizing gas has been discharged. It is also possible to transfer the liquid crude product mixture from the reactor of step (a) to another vessel, also called stripper, in which inert gas is passed through the mixture. For such a stripper, the above description of the continuous preparation process applies in a similar manner, except that the liquid crude product mixture is typically the stationary phase.

Inert gases suitable for stripping are substances which are gaseous under the conditions under which the stripping is carried out, which do not react with the liquid crude product mixture and are substantially free of molecular oxygen. Although the permissible concentration of molecular oxygen in the inert gas may depend on the saturated aliphatic C _6-12 The nature of the carboxylic acid and the conditions under which the liquid crude product mixture is stripped, but the inert gas preferably has a molecular oxygen content of 5 ppm by volume or less, more preferably 2 ppm by volume or less, particularly preferably 1ppm by volume or less. For example, small amounts of molecular oxygen may be present in the inert gas as impurities from their preparation or purification, respectively. Mention may be made, as possible inert gases, of nitrogen, hydrogen, carbon dioxide, carbon monoxide, nitrous oxide and the noble gases such as helium, neon, argon and krypton. The inert gas may be provided in pure form as well as in a mixture of two or more different inert gases. Nitrogen is preferred due to its availability.

The stripping in step (b) can be carried out at a wide range of temperatures and pressuresWithin the force range. Preferably at a temperature of from 0 to 150 ℃ and a pressure of from 0.0001 to 10MPa abs. More preferably, it is carried out at a temperature within the range of the temperature at which the liquid crude product mixture leaves the reactor unit in step (a) and the temperature at which the stripped mixture is fed to the distillation unit in step (c). The stripping is particularly preferably carried out at a temperature of ≥ 25 deg.C, particularly preferably ≤ 100 deg.C, very particularly preferably ≤ 60 deg.C, which is characteristic of the liquid saturated aliphatic C _6-12 The crude carboxylic acid product mixture is particularly advantageous in that undesired reactions between molecular oxygen and liquid mixtures, in particular already formed carboxylic acids, are avoided or at least minimized. With respect to pressure, it is more preferred to operate at a pressure within the range of the pressure at which the liquid crude product mixture leaves the reactor unit in step (a) and the pressure at which the stripped mixture is fed to the distillation unit in step (c). The wide pressure range indicated above has shown great flexibility in the choice of pressure. Depending on the facilities and infrastructure available at the plant site, the advantages of stripping under vacuum conditions or at atmospheric and higher pressures may prevail. In the case of sufficient vacuum power, it is generally preferred to be in the range of 0.0001 to<Stripping was carried out under vacuum conditions of 0.1MPa abs, because of the low amount of stripping gas required and the high stripping efficiency. Alternatively, stripping at abs > 0.1MPa has the advantage of not requiring vacuum power. However, stripping at atmospheric pressure or higher is preferably carried out at ≦ 0.2MPa abs, since the volume of inert gas is still sufficiently high to bubble particularly advantageously through the liquid mixture.

The amount of inert gas used to remove a specific amount of molecular oxygen by stripping can vary within wide limits. It has surprisingly been found that stripping is highly effective and that a low molar amount of inert gas per molar amount of molecular oxygen to be removed is sufficient. The stripping in step (b) is therefore preferably carried out at a molar ratio between the inert gas fed to the stripper and the molecular oxygen to be removed of ≥ 0.25 mol/mol. However, the molar ratio between the inert gas fed to the stripping column and the molecular oxygen to be removed is preferably not more than 1mol/mol, more preferably not more than 4mol/mol. The amount of inert gas used per unit time should be sufficiently low not to exceed the flooded point of the stripper. The person skilled in the art is able to calculate or experimentally determine the maximum permissible amount of inert gas per unit time in each case based on the geometry of the stripping column, the amount of liquid crude product mixture fed to the stripping column in the case of continuous operation, the process conditions such as temperature and pressure, and the physical properties of the mixture.

Based on the above-described low molar ratio between the inert gas fed to the stripping column and the molecular oxygen to be removed, stripping can generally be carried out in a continuously operated stripping column within 1 to 10 minutes, preferably for 2 minutes or more, preferably for 5 minutes or less. In batch operations, in which the liquid crude product mixture has been placed in the apparatus and then stripped therein with an inert gas, more time is generally required. For example, stripping in a bubble column operated batchwise typically takes several minutes to even hours to achieve the desired molecular oxygen removal.

By means of the invention, molecular oxygen is removed from the liquid mixture obtained in step (a), the content of molecular oxygen being reduced in step (b) to 10 ppm by weight or less, preferably 5 ppm by weight or less, more preferably 2 ppm by weight or less, particularly preferably 1ppm by weight or less, very particularly preferably 0.5 ppm by weight or less. The molecular oxygen content can even be reduced below its detection limit, but small amounts of > 0.05 ppm by weight still generally remain. The concentration of molecular oxygen in the liquid mixture obtained in step (b) can be easily determined in the same manner as already described in step (a).

The liquid obtained in step (b), saturated aliphatic C depleted in molecular oxygen, is then subjected to _6-12 The crude carboxylic acid product mixture is transferred to step (c). For the sake of completeness, reference to transfer may include further steps between steps (b) and (c), for example a reduction or an increase in temperature or pressure.

In step (c), the mixture obtained in step (b) is distilled in a distillation apparatus comprising a purification column, and a purification distillate containing 95% by weight or more of a saturated aliphatic carboxylic acid is withdrawn therefrom. The term purification column refers to a column in which saturated aliphatic C is obtained _6-12 A distillation column for purifying distillate of carboxylic acid. The distillation apparatus may comprise, in addition to the purification column, further distillation columns, for example a distillation column for removing light boiler components or a column for working up high boilers. Distillation column assemblyThe number is not limited, but usually 1 to 5, preferably 1 to 4, more preferably 1 to 3, particularly preferably 1 to 2 distillation columns are used. In addition to the distillation columns themselves and their internals, the term distillation apparatus also includes their peripheries, such as pipes, heat exchangers, reboilers, condensers, reflux drums, and the like.

Saturated aliphatic C _6-12 The distillative separation of the carboxylic acid can generally be carried out continuously or discontinuously.

In the case of a discontinuous or semi-continuous oxidation process in step (a), it is often advantageous to also carry out steps (b) and (c) discontinuously. For such discontinuous distillation, the distillation apparatus preferably contains only one distillation column which at the same time constitutes a purification column. Wherein low boilers are first separated off at the top of the column and saturated aliphatic C is subsequently obtained as a further fraction _6-12 A carboxylic acid. It may also be advantageous to carry out the removal of molecular oxygen and the subsequent distillation in the same distillation apparatus, in order to minimize the number of required equipment and simplify the process.

In the case of a continuous oxidation process in step (a), continuous and discontinuous removal of molecular oxygen in step (b) and distillation in step (c) may be advantageous and both are essentially equally preferred. However, the advantages of continuous distillation are generally applicable to greater throughput, while the advantages of discontinuous distillation are generally applicable to lesser throughput. In general, saturated aliphatic C for annual production _6-12 The plant with a yield of carboxylic acid of > 1000 tons is preferably a continuous distillation, while the plant with a corresponding yield of < 1000 tons per year is preferably a discontinuous distillation. Whereas for discontinuous distillation, the distillation apparatus preferably contains only a single distillation column, continuous distillation is usually carried out in a single distillation column or in an interconnection of a plurality of distillation columns. With regard to discontinuous distillation, the explanations in the preceding paragraphs apply accordingly. In the case of continuous distillation, this can be carried out, for example, in a distillation column which forms precisely the purification column and in which the low boilers are separated off at the top and the saturated aliphatic C is distilled off _6-12 The carboxylic acid is withdrawn as a side stream. The high boilers are removed as a bottom stream. The continuous distillation may also be carried out in two or more interconnected distillation columns. If two interconnected distillation columns are used, the low boilers are usually separated off in the first distillation column and comprise saturated aliphatic C _6-12 The carboxylic acid bottoms and the high boilers are transferred to a second distillation column which constitutes a purification column and in which the aliphatic C is saturated _6-12 The carboxylic acid is taken off overhead leaving the high boilers as bottoms. In a further and disadvantageous variant with two interconnected distillation columns, the saturated aliphatic carboxylic acid is discharged together with the low boilers at the top of the first distillation column and is therefore already separated from the high boilers in the first distillation column, but then purification needs to be carried out in the second distillation column, which would require further energy-intensive evaporation of the saturated aliphatic carboxylic acid. Instead of using two interconnected distillation columns, a dividing wall column or equivalent Petlyuk configuration can be used. For the sake of completeness, reference is also made to the use of three or more distillation columns, for example in order to also separate light boilers into different fractions. If two or more different saturated aliphatic C's are prepared together _6-12 The use of three or more distillation columns of carboxylic acids may also be of interest, since for each saturated aliphatic C _6-12 Carboxylic acid this usually requires a separate purification column.

Whether only one distillation column is used or two or more distillation columns are interconnected, they can be and preferably are equipped with internals which promote the separation efficiency, for example structured packing, random packing or trays. The number of separation stages required depends mainly on the separation task, in particular the difference between the boiling point of the saturated aliphatic carboxylic acid and the boiling points of the low and high boilers, and the target purity of the saturated aliphatic carboxylic acid.

As regards the operating conditions in step (C), it is advisable not to use saturated aliphatic C _6-12 The carboxylic acid is exposed to temperatures in excess of 170 c because it is recognized according to the invention that higher temperatures are increasingly beneficial in the formation of unwanted by-products, such as anhydrides. Thus, processing saturated aliphatic C _6-12 The temperature in the distillation column for the carboxylic acid is preferably 170 ℃ or less, in particular 170 ℃ or less, more preferably 150 ℃ or less, and particularly preferably 130 ℃ or less. This can be easily achieved by choosing the appropriate pressure. Due to even the lowest boiling saturated aliphatic C _6-12 The boiling point of the carboxylic acid at atmospheric pressure is also above 190 ℃ and the distillation in step (c) is preferably carried out at 0.1 to 99kPa abs, which involves the treatment of saturated aliphaticC _6-12 All distillation columns for carboxylic acid. The distillation is more preferably carried out at a pressure of not less than 0.5kPa abs, particularly preferably not less than 1kPa abs, still more preferably not less than 50kPa abs, and particularly preferably not less than 20kPa abs. If more than one distillation column is used, each distillation column may be operated at a different pressure. In view of the above pressure ranges, the distillation in step (c) is generally carried out at a temperature of 0 ℃ or more, preferably 25 ℃ or more, more preferably 40 ℃ or more.

Distillation at very low pressures may be to obtain particularly pure saturated aliphatic C's since very low pressures of 1kPa abs result in low distillation temperatures of typically 100 ℃ or less, and such low distillation temperatures significantly reduce the detrimental effects of molecular oxygen _6-12 A particular choice of carboxylic acid, or possibly one that tolerates a slightly higher molecular oxygen content. However, this very low pressure variant is not a preferred option because at these very low pressures the distillation rate drops rapidly, resulting in very low throughput or the need for disadvantageously large diameter columns.

Based on the above findings and relationships, the purification column is preferably operated at a pressure of 0.1 to 99kPa abs and a temperature of 0 to 170 ℃.

As a logical result of the surprising discovery, it is possible to avoid or at least significantly reduce the darkening of carboxylic acids over time during storage, when subjected to thermal stresses and/or in the products in which they are conventionally used, if the liquid saturated aliphatic C obtained in step (a) is _6-12 The molecular oxygen present in the crude carboxylic acid product mixture is removed in step (b) to a content of < 10% by weight, based on the liquid mixture, and the saturated aliphatic C is separated off by distillation _6-12 The carboxylic acid, of course, after removal of molecular oxygen in step (b), including transfer to step (c), will react during or after distillation in step (c). If saturated aliphatic C is treated _6-12 The carboxylic acid plant is sealed or at least largely sealed under operating conditions, and the permeation of molecular oxygen can be avoided or at least reduced to a very low degree. This is particularly desirable for components operating under vacuum, particularly for distillation columns, and especially for purification columns. May be carried out by various techniques known to those skilled in the artThe surgical procedure achieves a high tightness of the column used, in particular of the vacuum distillation column and of the stripping column (if it is operated under reduced pressure). For example, these include using high quality seals and even welded joints with low leakage rates, flushing the flange protector with inert gas, or minimizing the number of flanges. Generally, the more such means are implemented, the lower the leakage rate of the distillation column.

Although it is desirable to avoid any permeation of molecular oxygen in step (C), the saturated aliphatic C is depleted with respect to the molecular oxygen treated in step (C) _6-12 The low amount of permeated molecular oxygen does not significantly impair the saturated aliphatic C in respect of the amount of the crude carboxylic acid product mixture _6-12 The purity of the carboxylic acid distillate and generally still allows to obtain a low APHA color number within the specified range after the tempering procedure specified in step (c). Thus, for example, a large distillation column of technical scale designed for distillation of 1000kg or more per hour is generally able to withstand a greater absolute amount of permeated molecular oxygen per unit time than a small laboratory scale distillation column of laboratory scale, which only processes small amounts of, for example, 1kg or less per hour. Furthermore, the ratio between the seal surface and the internal volume of the distillation column is much lower for large columns on an industrial scale than for small laboratory columns. According to the invention, it is recognized that the amount of molecular oxygen permeated into the distillation unit of step (C) is in the saturated aliphatic C depleted in molecular oxygen by step (b) _6-12 The amount of molecular oxygen supplied to the distillation apparatus from the crude carboxylic acid product mixture is tolerable in the range.

Although saturated aliphatic C is treated therein _6-12 All the parts of the distillation apparatus of step (C) of the carboxylic acid should advantageously be sealed as reasonably as possible, in particular the tightness of the purification column against saturated aliphatic C _6-12 The purity of the carboxylic acid distillate is more sensitive. This applies in particular to purification columns operating under vacuum conditions. The tightness or in other words the leak rate of the distillation column can be determined by measuring the amount of molecular oxygen permeated during the distillation and by comparing these values with the amount of molecular oxygen fed to the distillation column via the feed stream. For a vacuum tower, this can be done by measuring the amount of off-gas in the vacuum unit and the concentration of molecular oxygen thereinAnd (4) obtaining.

It is preferred that the purification column, which is operated at a pressure of from 0.1 to 99kPa abs and a temperature of from 0 to 170 ℃, is at least so sealed that the amount of molecular oxygen withdrawn from the off-gas of its vacuum unit is 2 times or less the amount of molecular oxygen fed to the distillation apparatus from the mixture obtained in step (b). More preferably, the amount of molecular oxygen taken off from the offgas of the vacuum unit thereof is ≦ 1.8, particularly preferably ≦ 1.5, and with respect to the lower limit ≧ 1 time the amount of molecular oxygen fed to the distillation apparatus from the mixture obtained in step (b). With regard to discontinuous distillation, the term amount represents an absolute amount during discontinuous distillation, whereas with regard to continuous distillation, the term amount relates to an amount per unit time. Saturated aliphatic C depleted in molecular oxygen fed to a distillation apparatus _6-12 The molecular oxygen content of the crude carboxylic acid product mixture can be readily determined by measuring the molecular oxygen concentration as described above, for example by using an optical molecular oxygen sensitive sensor and taking into account the total amount of product mixture in the case of discontinuous distillation or correlating it with the amount of product mixture fed to the purification column per unit time in the case of continuous distillation. The amount of molecular oxygen withdrawn from the off-gas of the vacuum unit of the purification column can be easily determined by measuring the molecular oxygen concentration of the off-gas of the vacuum unit, preferably on the pressure side, and by determining the amount of off-gas, for example using a flow meter. For discontinuous distillation, the total amount is calculated, and for continuous distillation, the relative amount per unit time is calculated.

Since the above-described method of determining the leak tightness or leakage rate of a vacuum tower requires the amount of off-gas in addition to the concentration of molecular oxygen in the off-gas of the tower, it may sometimes be difficult to obtain a reliable value of the amount of off-gas, since its amount is sometimes very low and therefore difficult to measure. Therefore, an alternative measurement method that does not require the amount of column off-gas has been developed. This alternative method takes into account that a vacuum tower that is not absolutely sealed will draw some ambient air into the tower. Since air contains a fixed ratio of argon to molecular oxygen, the concentration of argon in the column off-gas can be used as an indirect measure of the molecular oxygen concentration based on air permeation. The molecular oxygen concentration in air was 20.95 vol%, the argon concentration was 0.93 vol%, corresponding to an Ar/O of 0.0444 ₂ The molar ratio. Feed material isSaturated aliphatic C, depleted in argon and in molecular oxygen _6-12 Absolute sealed column of crude carboxylic acid product mixture results in Ar/O ₂ The molar ratio was 0 (zero) because no argon was present in the column off-gas. In contrast, a blow-by column feeding a product mixture free of argon and molecular oxygen results in Ar/O ₂ The molar ratio was 0.0444, which is the theoretical value for air. Depending on the amount of molecular oxygen fed to the distillation column together with the product mixture, ar/O for the leakage column ₂ The molar ratio will be lower than for pure air. Ar/O is the same if the amount of permeated molecular oxygen is the same as the amount of molecular oxygen fed to a distillation column having a product mixture depleted in molecular oxygen ₂ The molar ratio will be 0.0222. The more sealed the tower, the Ar/O ₂ The lower the molar ratio and vice versa, whereas a molecular oxygen concentration of 0% by volume in the column off-gas is, by definition, relevant for an absolutely sealed distillation column.

According to this alternative determination method, the purification column, which is preferably operated at a pressure of 0.1 to 99kPa abs and a temperature of 0 to 170 ℃, is at least so sealed that the molar ratio n (Ar off-gas)/n (O) in the off-gas of the vacuum unit of the purification column is such that ₂ Off-gas) of 0 to 0.0222, wherein n (Ar off-gas) is the molecular weight of argon in the off-gas, corrected by the molecular weight of argon possibly fed to step (c) from the mixture obtained in step (b), and n (O) ₂ Exhaust gas) is the molecular weight of molecular oxygen in the exhaust gas. More preferably, the molar ratio n (Ar off-gas)/n (O) ₂ Waste gas) of 0.0197 or less, and particularly preferably 0.0148 or less. For discontinuous distillation the term amount refers to the absolute amount during discontinuous distillation, whereas for continuous distillation the term amount refers to the amount per unit time. With regard to the measurement of molecular oxygen in the exhaust gas of the vacuum cell, the above-described measurement method can be applied. The molecular weight of argon in the off-gas of the vacuum cell can be easily determined, for example, by gas chromatography analysis of the off-gas on the pressure side. For example, a portable gas chromatograph may be used. If the mixture obtained in step (b) already contains argon, the amount thereof can also be determined by gas chromatography and used for the correction of the n (Ar off-gas) value, for whatever reason.

Further, it is recognized in accordance with the invention that the location of the feed point for feeding feed to the purification column relative to the removal of saturated fatsGroup C _6-12 The location of the carboxylic acid sampling point also affects the saturated aliphatic C _6-12 The tempering stability of the carboxylic acid, especially if the distillation column is not a dividing wall column. If the position of the feed point of the distillation column other than the dividing wall column is higher than the position of the sampling point, the saturated aliphatic C _6-12 The tempering stability of carboxylic acids is generally lower than the opposite. It is therefore preferred to add the feed to a purification column which is not a dividing wall column in step (c) at a point below the withdrawal point of the purified distillate. Taking the total external height of the purification column as 100%, the distance between the level of the feed point and the level of the sampling point which is higher than the level of the feed point is preferably ≥ 1%, more preferably ≥ 2%, particularly preferably ≥ 3%, and preferably ≤ 90%, more preferably ≤ 80%, based on the external height of the purification column. For the dividing wall column as a purification column, the relative positions of the feed point and the sampling point are opposite to those of the saturated aliphatic C _6-12 The tempering stability of the carboxylic acid is less affected.

Based on the above description of the distillation apparatus and its operation, the person skilled in the art can rely on saturated aliphatic C _6-12 The nature of the carboxylic acid is designed to the distillation apparatus and to determine the appropriate operating conditions.

The above process enables the preparation of saturated aliphatic C having a content of 95% by weight or more based on the distillate in high purity _6-12 Saturated aliphatic C of carboxylic acid and having an APHA colour number which is extremely low even after tempering under inert gas conditions at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours _6-12 A carboxylic acid. Expressed in numbers, a purified distillate containing 95% by weight or more of saturated aliphatic carboxylic acid based on the distillate has an APHA color number of 0 to 10 after tempering at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours under inert gas conditions. Saturated aliphatic C _6-12 The content of carboxylic acid is preferably 98% by weight or more, more preferably 99% by weight or more, particularly preferably 99.5% by weight or more, very particularly preferably 99.8% by weight or more, based on the distillate. As typical by-products, formate esters, carboxylic acids having a lower number of carbon atoms, alcohols, ketones and water are mentioned. For the sake of clarity, reference to APHA color numbers refers to Hazen shades established and well known according to DIN EN ISO 6271 and ASTM D1209, respectively.

Freshly distilled, unreturned, according to the low APHA color number after the above tempering procedureSaturated aliphatic C of fire _6-12 The APHA color number of the carboxylic acids is also very low, generally from 0 to 10, preferably 5 or less, more preferably 3 or less.

Saturated aliphatic C _6-12 The above tempering procedure of carboxylic acids at a temperature of 225 ℃ and a pressure of 0.1MPa under inert gas conditions for 4 hours simulates the thermal stress scenario under defined and reproducible conditions and is directly a more or less reliable indication of the color stability when subjected to thermal stress during storage and/or after routine use in the manufacture of products.

In more detail, the so-called tempering test is carried out as follows: 40.0 to 60.0g of a saturated aliphatic C-containing compound _6-12 A sample of the carboxylic acid was placed in a three-necked round bottom flask equipped with a magnetic stirrer, condenser, and a glass frit for bubbling an inert gas (about 0.2Nl/min, preferably argon 5.0) through the sample. At ambient temperature, the sample was bubbled with an inert gas for 30 minutes while stirring. Bubbling was then minimized and the flask lowered into a pre-heated oil bath maintained at 225 ℃. After 4 hours, the bubbling rate was slightly increased and the heating bath was removed. It is important that the bubbling rate be high enough to prevent air from being drawn into the flask during cooling. When the flask cooled to ambient temperature, a sample was taken and the APHA color number was measured. For example, a 10mL cuvette in Hach may be used

The measurement is performed in a color comparator 620. The instrument was calibrated beforehand with distilled water.

Purified saturated aliphatic C after tempering according to the above tempering test _6-12 The APHA color number of the carboxylic acid distillate is preferably 8 or less, more preferably 5 or less. Although the APHA color number may reach 0, it is usually ≧ 1.

Due to the saturated aliphatic C obtained in step (C) _6-12 The discoloration of the carboxylic acid distillate in the tempering test is mainly caused by the presence of small amounts of peroxides, the untempered saturated aliphatic C _6-12 The active oxygen content in the carboxylic acid distillate is another indicator of its color stability. According to the invention, the active oxygen content of the purified distillate obtained in step (c) is preferably from 0 to 100 ppm by weight, more preferably < 50 ppm by weight, particularly preferably < 10 ppm by weightppm, based on the distillate, preferably ≥ 1ppm by weight.

As previously mentioned, each of steps (a) - (c) may be performed continuously or discontinuously, while semi-continuous operation is an additional option for step (a). It may be advantageous for lower throughputs to produce the saturated aliphatic C in a discontinuous or semi-continuous process _6-12 Carboxylic acids because such discontinuous and semi-continuous processes are generally more flexible and easier to operate for small quantities. On the other hand, a continuous process in which steps (a) - (c) are carried out continuously has the advantage of more efficient and stable operation once the process is started. Therefore, for>Higher capacity of 1000 tons/year, the preferred option being to continuously prepare the saturated aliphatic C _6-12 A carboxylic acid. This applies in particular to saturated aliphatic carboxylic acids which are produced in large amounts, such as octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid and dodecanoic acid.

In a general embodiment for the continuous preparation of 2-ethylhexanoic acid, liquid 2-ethylhexanoic aldehyde, an aqueous solution of potassium hydroxide as selectivity-improving additive and gaseous oxygen are continuously fed into a jet loop reactor operated at a temperature in the range of from 30 to 60 ℃ and an oxygen partial pressure in the range of from 0.11 to 1MPa abs. In order to remove the heat generated by the exothermic oxidation reaction, the reactor is cooled externally by an external heat exchanger. The reaction mixture obtained by step (a) described above still contains unconverted 2-ethylhexanoal in an amount of 0.1-2mol% relative to 2-ethylhexanoic acid and molecular oxygen in a concentration of 20-500 weight ppm, depending on the partial pressure of oxygen, the temperature and the conversion based on the liquid mixture. It is fed to a stripping column and stripped with a nitrogen stream in an amount of 0.5 to 4mol of nitrogen per mol of molecular oxygen at 40 to 60 ℃ and about atmospheric pressure or vacuum to obtain a mixture which is depleted in molecular oxygen by less than or equal to 10 ppm by weight, based on the liquid mixture. The mixture depleted in molecular oxygen is then continuously fed to a distillation apparatus operating under vacuum, which contains a low boiler column for separating low boilers such as water, residual 2-ethylhexanoic aldehyde and other low-boiling by-products and a purification column which is fed with the bottom stream of the low boiler column and in which purified 2-ethylhexanoic acid is discharged as a side stream. Optionally, the two columns can also be combined into a single dividing wall column. The 2-ethylhexanoic acid obtained in the above distillation step (c) has a high purity, and contains not less than 99.5% by weight of 2-ethylhexanoic acid. Only an APHA color number of 10 or less was shown after tempering at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours under inert gas conditions.

In a general embodiment for the discontinuous preparation of 2-ethylhexanoic acid, 2-ethylhexanoic acid as the dilution solvent and potassium hydroxide as the selectivity-improving additive are placed in a temperature-controlled stirred vessel, while 2-ethylhexanoic aldehyde and molecular oxygen or a gas mixture containing molecular oxygen are fed into the stirred vessel at a total pressure of 0.1-10MPa abs during 0.1-5 hours, while the temperature is controlled at 25-60 ℃. After the desired level in the stirred vessel was reached, the addition of 2-ethylhexanal was stopped and molecular oxygen was replenished to maintain the partial pressure and to bring the conversion to the desired value. After the desired conversion has been reached, the addition of molecular oxygen is also stopped, and instead of nitrogen, stripping is carried out at 40-60 ℃ and about atmospheric pressure or vacuum while continuing stirring in the vessel. Typically, a nitrogen stream of 0.5 to 100mol of nitrogen per mol of dissolved molecular oxygen is fed in over a period of 10 minutes to 2 hours, in order to obtain a mixture depleted in molecular oxygen of 10 ppm by weight or less, based on the liquid mixture. The mixture depleted in molecular oxygen is then placed in a batch-type vacuum distillation column and, after removal of the low boilers, a fraction containing purified 2-ethylhexanoic acid is obtained or fed continuously to a continuously operated distillation apparatus and purified 2-ethylhexanoic acid is removed therefrom. The obtained purified 2-ethyl hexanoic acid has high purity and contains more than or equal to 99.5 weight percent of 2-ethyl hexanoic acid. Only an APHA color number of 10 or less was shown after tempering at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours under inert gas conditions.

In addition to the above process, 2-ethylhexanoic acid having an APHA color number of 0-10 is obtained after tempering at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours under inert gas conditions. This is surprising since even 2-ethylhexanoic acid is produced by a process other than the oxidation process by the processes described in the prior art, for example by dehydrogenation of 2-ethylhexanol with NaOH to sodium 2-ethylhexanoate and subsequently with H ₂ SO ₄ Acidification, although it is not expected that peroxides will form in the process, will not produce a recycle meeting the aboveFire tested temper stable 2-ethylhexanoic acid.

The process of the present invention enables the preparation of saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxidation of the corresponding aldehydes with oxygen in high yield and purity, in particular with high color stability, so that the APHA color number is low even after tempering for 4 hours at a temperature of 225 ℃ and a pressure of 0.1MPa under inert gas conditions. Saturated aliphatic C _6-12 Carboxylic acids also have very low levels of active oxygen and therefore very low levels of peroxides such as peroxy acids, hydroperoxides and other peroxides. The process can also be operated easily and stably over a long period of time to produce the saturated aliphatic carboxylic acid at a constantly high quality. Saturated aliphatic C _6-12 The carboxylic acids can be stored for long periods, heat exposed or applied in the production of other products without darkening or at least with a very low tendency to darken.

In addition, the method can obtain 2-ethyl hexanoic acid with high purity and stable tempering.

Examples

Determination of active oxygen by iodometry

The content of active oxygen in the sample was determined by iodometry. A general description of how the assay is performed is given below.

Approximately 5g of the sample weighed to the nearest 0.1mg was placed in a reaction flask, purged with argon, and 40ml of a1 acetic acid/chloroform mixture was added to dissolve the sample. The reaction flask was equipped with a cooler and placed in a stirred heating block preheated to 80 ℃. A weak argon stream was passed through the cooler to prevent air ingress. After temperature equilibration, 5.0ml of saturated potassium iodide solution (approximately 60.0g of potassium iodide dissolved in 100ml of deionized water) was added via a cooler, and the mixture was boiled under reflux for 10 minutes. In the next step, 40.0ml of deionized water was added and the sample solution was titrated with a 0.01M thiosulfate solution while using a platinum electrode as an end point indicator.

Determination of molecular oxygen

By high precision and high O suitable for measuring molecular oxygen in carboxylic acids ₂ A sensitive optical fluorescence sensor measures the molecular oxygen content in a sample. In the examplesIn using the name from WTW

925.

The measured data were cross-checked by additional measurements using a galvanic oxygen analyzer (also known as a Hersch cell).

Determination of APHA color number

The APHA color number of the samples was determined by a colorimeter calibrated against distilled water in advance. In the examples, the name from Hach with a 10mL cuvette was used

620.

Description of the tempering test

The so-called tempering test simulates the thermal stress scenario under defined and reproducible conditions to show the color stability of the samples during storage, under thermal stress and/or after routine use in the manufacture of products. A description of how the tempering test is performed is given below.

40.0-60.0g of the sample was placed in a three-necked round bottom flask equipped with a magnetic stirrer, condenser, and a glass frit for bubbling an inert gas (about 0.2Nl/min, argon 5.0) through the sample. At ambient temperature, the sample was bubbled with an inert gas for 30 minutes while stirring. Bubbling was then minimized just to prevent air from being drawn into the flask, and the flask was lowered into a pre-heated oil bath maintained at 225 ℃. After 4 hours, the bubbling rate was slightly increased and the heating bath was removed. The slightly increased bubbling rate must be high enough to prevent air from being drawn into the flask during cooling. When the flask cooled to ambient temperature, a sample was removed and the APHA color number was measured as described above.

Example 1 (preparation of crude 2-ethylhexanoic acid)

Crude 2-ethylhexanoic acid is produced according to step (a) of the present invention. 2-ethylhexanoic acid is produced by continuous oxidation of 2-ethylhexanoal with pure oxygen in a technical installation with a capacity of about 3.75 tons of 2-ethylhexanoic acid per hour at a temperature of 30-60 ℃ and a pressure of 0.25MPa abs and in the presence of 0.4% by weight of potassium ions in the reaction mixture. The technical installation contains three reactors in series and the addition of the oxygen feed is divided between the three reactors. 2-ethylhexanal and potassium salt were added to the first reactor.

The crude 2-ethylhexanoic acid obtained at the outlet of the third reactor contained 92.4% by weight of 2-ethylhexanoic acid and 281 ppm by weight of active oxygen. The APHA color number was determined to be > 1000. The molecular oxygen content at the outlet of the third reactor before depressurization was 600 ppm by weight. Even after cooling to ambient temperature and depressurization to ambient pressure, the crude 2-ethylhexanoic acid still contains 210 ppm by weight of molecular oxygen. The measured values are summarized in table 1.

Example 2 (comparative example)

A1 kg sample of crude 2-ethylhexanoic acid from example 1 was distilled in a pilot scale batch still containing a 2m column packed with a meshed ring. The column was operated at an overhead pressure of 1kPa abs, 2-ethylhexanoic acid was distilled off at the top of the column and the fraction boiling at 102.5. + -. 0.5 ℃ was collected. The recovered 2-ethylhexanoic acid was analyzed by gas chromatography to have a 2-ethylhexanoic acid content of 99.38 wt.% and contained 11 wt ppm of active oxygen. The APHA color number was 2. The measured values are summarized in table 1.

The distilled samples were then tempered under the tempering test conditions described above and the APHA color number was measured. The APHA color number after the tempering test was 40. This high value indicates that the color stability of 2-ethylhexanoic acid treated according to the prior art is far from adequate.

Example 3 (example according to the invention)

Another sample of 1kg of crude 2-ethylhexanoic acid from example 1 was first stripped with high purity nitrogen (purity grade 5.0) at ambient temperature and pressure until the molecular oxygen content was only 2 ppm by weight. The procedure relates to step (b) of the present invention. This sample was then distilled in the same batch distillation apparatus used in example 2, containing a 2m column packed with a reticulated ring, with special care being taken to minimize air leakage into the column by carefully lubricating all glass joints with high vacuum silicone grease. The column was operated at an overhead pressure of 1kPa abs, 2-ethylhexanoic acid was distilled off at the top of the column and the fraction boiling at 102.5. + -. 0.5 ℃ was collected. The recovered 2-ethylhexanoic acid had a 2-ethylhexanoic acid content of 99.99% by weight as analyzed by gas chromatography and contained only 2 ppm by weight of active oxygen. Its APHA color number is 0 and the active oxygen content is below the detection limit (< 1 ppm). The measured values are summarized in table 1.

The distilled samples were then tempered under the tempering test conditions described above and the APHA color number was measured. The APHA colour number after the tempering test was only 7 (according to step (c)). This low value indicates that the removal of molecular oxygen in step (b) to less than or equal to 10 ppm by weight of the present invention enables the preparation of highly color-stable 2-ethylhexanoic acid.

Example 4 influence of molecular oxygen on the APHA color number after tempering test

500ml of a sample of purified 2-ethylhexanoic acid obtained in example 3 was charged into a 1-liter flask and immersed in a tempering bath at 40 ℃ without any precautions to prevent contact with air. Starting at time "0", a stream of 10NL/h air was bubbled through the sample via the frit. And (4) sampling periodically, and analyzing the content of active oxygen and molecular oxygen and the APHA color number of the active oxygen and the molecular oxygen before and after the tempering test. The results are summarized in table 2.

Since some time was required to transfer the sample of example 3 to the experimental setup of example 4, the content of molecular oxygen increased from <1 wt ppm to 14 wt ppm at the beginning of experiment 4, which is represented by time "0". Over time, the molecular oxygen content increased to 54 ppm by weight after 5 hours (sample No. 3). With this increase, the active oxygen content after the tempering procedure and the APHA color number also increased, while the APHA color number of the untempered samples remained at the 0 value. An increase in the active oxygen content indicates that a portion of the molecular oxygen is converted to active oxygen, since molecular oxygen itself cannot be measured iodometrically. After 5 hours (sample 3), the molecular oxygen content was maintained at a constant value of 54 ppm by weight, which is the expected saturation concentration of molecular oxygen in 2-ethylhexanoic acid. Nevertheless, the active oxygen content and APHA color number after the tempering test still increased with increasing time of air bubbling through the sample. This indicates that molecular oxygen is continuously converted to reactive oxygen and, together with molecular oxygen, leads to a high APHA color number after the tempering procedure.

Example 5 (Effect of molecular oxygen depletion on APHA color number after tempering test)

According to example 4, the air sparging was stopped after 24 hours (i.e., after sampling sample No. 5) and replaced by sparging with 10NL/h nitrogen (purity grade 5.0) for 24 hours. After 24 hours of nitrogen bubbling, a sample designated sample No. 6 was sampled and analyzed as in example 4.

Bubbling with nitrogen had little effect on the active oxygen content, which only slightly decreased from 31 wt ppm in sample No. 5 to 5-28 wt ppm in sample No. 6. However, the amount of molecular oxygen was significantly reduced from 64 wt% in sample No. 5 to only 14 wt ppm in sample No. 6. This is an approximately 4-fold decrease. Thus, the APHA color number after the tempering test also decreased from 112 in sample No. 5 to 71 in sample No. 6. This indicates that the APHA color number after the tempering test depends on both the molecular oxygen content and the active oxygen content. With the significant decrease in molecular oxygen content in sample No. 6, the APHA color number after the tempering test also decreased.

Table 1: examples 1 to 3

Claims

1. A process for preparing saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxidation of the corresponding aldehyde with molecular oxygen, comprising:

(a) Converting the corresponding aldehyde with molecular oxygen at a temperature of from 0 to 120 ℃ and an oxygen partial pressure of from 0.02 to 2MPa to obtain a liquid mixture containing a saturated aliphatic carboxylic acid and less than or equal to 2mol% of the corresponding aldehyde relative to the saturated aliphatic carboxylic acid and molecular oxygen;

(b) Removing molecular oxygen from the liquid mixture obtained in step (a) to a content of 10 ppm by weight or less, based on the liquid mixture; and

(c) Distilling the mixture obtained in step (b) in a distillation apparatus comprising a purification column and withdrawing therefrom a purified distillate containing 95% by weight or more, based on the distillate, of a saturated aliphatic carboxylic acid.

2. The process according to claim 1, wherein the saturated aliphatic carboxylic acid is 2-ethylhexanoic acid and the aldehyde is 2-ethylhexanal.

3. The process according to claim 1 or 2, wherein the liquid mixture obtained in step (a) has a content of molecular oxygen of >10 ppm by weight to ≦ 1% by weight, based on the liquid mixture.

4. A process according to any one of claims 1 to 3, wherein molecular oxygen is removed in step (b) by stripping with an inert gas.

5. The process according to claim 4, wherein the production of the saturated aliphatic carboxylic acid is carried out continuously and the stripping is carried out in a countercurrent flow.

6. The process according to claim 4 or 5, wherein the inert gas has a molecular oxygen content of 5 ppm by volume or less.

7.A process according to any one of claims 4 to 6, wherein the removal of molecular oxygen in step (b) is carried out at a temperature of from 0 to 150 ℃ and a pressure of from 0.0001 to 10MPa abs.

8. The process according to any one of claims 1 to 7, wherein in step (b) molecular oxygen is removed in step (b) to 2 ppm by weight or less.

9. The process of any one of claims 1-8, wherein in step (c), the purification column is operated at a pressure of 0.1 to 99kPa abs and a temperature of 0 to 170 ℃.

10. The process according to claim 9, wherein the amount of molecular oxygen withdrawn from the offgas of the vacuum unit of the purification column is 2 times or less the amount of molecular oxygen fed from the mixture obtained in step (b) to the distillation apparatus.

11. The method according to claim 9, wherein the molar ratio n (Ar off-gas)/n (O) in the off-gas of the vacuum unit of the purification column ₂ Off-gas) of 0 to 0.0222, wherein n (Ar off-gas) is the molecular weight of argon in the off-gas, corrected by the molecular weight of argon possibly fed to step (c) from the mixture obtained in step (b), and n (O) ₂ Exhaust gas) is the molecular weight of molecular oxygen in the exhaust gas.

12. The process according to any one of claims 1 to 11, wherein the feed is added in step (c) to a purification column which is not a dividing wall column at a point below the withdrawal point of the purification distillate.

13. The process according to any one of claims 1 to 12, wherein the purified distillate obtained in step (c) contains 99.5% by weight or more of saturated aliphatic carboxylic acids.

14. The process of any one of claims 1 to 13, wherein the purified distillate has an APHA color number of from 0 to 10 after tempering under inert gas conditions at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours.

15. The process according to any one of claims 1 to 14, wherein the purified distillate obtained in step (c) has an active oxygen content of 0 to 100 ppm by weight, based on distillate.

16. Ethylhexanoic acid having an APHA color number of 0-10 after tempering under inert gas conditions at a temperature of 225 ℃ and a pressure of 0.1MPa for 4 hours.